GB1178693A - Improvements in or relating to Carrier Telephony Equipment - Google Patents

Abstract

1,178,693. Intercommunication systems; combined radio receiving and transmitting systems. GENERAL ELECTRIC & ENGLISH ELECTRIC COMPANIES Ltd. 2 June, 1967 [26 April, 1966], No. 18224/66. Headings H4K, H4L and H4R. In a carrier telephony intercommunication system each substation incorporates a local oscillator which is used both to frequency change incoming signals to an intermediate frequency to be amplified in an i.f. amplifier, and also as a carrier frequency for outgoing signals. The local oscillator in each substation is normally tuned to a frequency different from that of any other substation but is tunable to any one of a number of other frequencies to allow communication with any other substation on that substation's frequencies. Fig. 3 shows a substation circuit in which the local oscillator 22 comprises a self oscillating mixer tunable by push-buttons 1-20 to any one of 20 oscillator frequencies one of which is the normal call receiving frequency for the set and the remaining 19 allowing communication with any one of 19 similar sets. Incoming signals are fed from a coaxial line 21 to the base of mixer 23 through the tuned circuit 24, the i.f. signal being obtained in the collector circuit and fed via tuned transformer 26, i.f. amplifier 37, detector 38, and a.f. amplifier 39 to the receiver 40. Speech signals from the microphone 36 modulate the collector voltage of transistor 23, via the a.f. amplifier 35 and transformer 28, and cause the oscillatory output of the transistor to be amplitude modulated by the speech, the resulting signals being fed to the coaxial line via the output tapping on the oscillator coil 28. Normally, in a listening or called condition, the input circuit 28 is tuned to a frequency in a band from 985 to 1365 kc./s., at a frequency 470/kc./s., the i.f., below the output circuit 29, i.e. the oscillator or carrier output frequency, the different substations of one system being tuned to frequencies with 20 kc./s. spacing. On depression of a button to call another station the carrier output or oscillator frequency of the calling station is tuned to the input circuit frequency of the called station and the input circuit frequency of the calling station is tuned to a frequency 470 kc./s. above the oscillator frequency to be able to receive the carrier frequency output signals of the called stations. The calling buttons of each substation are arranged to have a non-locking overtravel which operates contacts 41 to connect the a.f. amplifier 35 as an oscillator to generate a calling signal, and each substation has a reed relay hook switch, see Specification 1,178,695, operated by a magnet in the hand set, to reduce the output level of the calling tone when the hand set is lifted, so as to prevent ear damage due to the high level at which the calling tone is produced.